1. Technical Field
The invention relates to pumps, in particular to positive-displacement pumps, for oil and other media, preferably liquids. In particular, the invention relates to pumps comprising means of limiting and/or varying delivery. One preferred field of application is in motorized land, air and water vehicles, in particular automobiles and heavy goods vehicles. However, pumps in accordance with the invention are also advantageously applicable in other fields, for example the hydraulic supply of a press.
2. Description of Related Art
In EP 0 994 257 A1 an external gear wheel pump is described, which varies the specific displacement, i.e. displacement/pump speed. This variation is achieved by altering the meshing length of two meshed gear wheels. For this purpose, one of the gear wheels is supported on a piston, receiving on one side the pressure of the pump and on the other side the pressure of a spring, opposing the pump pressure.
A fluid machine in the form of a vane pump including a magnetic clutch is known from EP 0 855 515 A1, for application as a governed motor vehicle coolant pump. The magnetic clutch is adjusted according to the rotational speed, as measured by a sensor, to deliver the coolant according to requirement. Adjustment is achieved by a servomotor and a mechanical gear wheel unit.
In gear wheel pumps, however, for example external and internal gear wheel pumps forming preferred examples of oil pumps in accordance with the invention, two gear wheels mesh and, together with the walls of a surrounding casing, form a displacement space through which the medium to be displaced is delivered, from a low pressure side to a high-pressure side of the pump. The low-pressure side is connected to an inlet port and the high-pressure side to an outlet port of the pump.
In known gear wheel pumps, one of the two gear wheels of a gear wheel set is supported by the casing of the pump. The other gear wheel is rotationally driven by a rotary drive member and is non-rotatably connected to the rotary drive member for this purpose. The rotary drive member supports this gear wheel. In general, the gear wheel is directly connected non-rotatably to the rotary drive member. The rotary drive member is in turn rotatably supported relative to the casing. For reasons of production tolerances, inaccuracies in assembly and loads occurring during operation, the rotary drive member xe2x80x9cworksxe2x80x9d relative to the casing. Accordingly, undesirable movements of the gear wheels of the gear wheel pump relative to each other, for example tilting, also arise.
Positive-displacement pumps, in particular gear wheel pumps, generally comprise a specific delivery [displacement/feed-wheel speed] which is constant according to the system involved, because the geometry of the displacement pockets cannot be altered. They show a proportionality of delivery to speed, as long as the filling ratio of the displacement pockets is 100%. However, in many applications this proportionality is disruptive and undesirable. In a press for example, although a high delivery of the hydraulic fluid is necessary for the rapid motion, only high pressure is required in the end phase of the working stroke, and the oil delivery requirement drops to zero. Since the drive speed of such pumps in presses remains as a rule constant, a high-pressure excess flow of oil arises, which is returned to the fluid reservoir afflicted with a loss of energy. Such an excess flow is particularly disruptive, for example, in automotive engine lube pumps and in automatic transmission fluid pumps. At low engine speeds and thus low pump speeds, these assemblies do require a minimum delivery when idling, and a minimum fluid pressure at high speed, however the flow requirement at high speed is well under the proportionality line, at top speeds mostly under a third of the proportionality flow.
It is an object of the invention to reduce noise and wear in pumps, preferably in oil pumps and hydrostatic pumps in general, said pumps having means for limiting or varying delivery, or both in combination.
This object is achieved by the subject matters of the independent claims. The sub-claims describe particularly preferred embodiments of pumps.
In accordance with the invention, a pump, preferably a gear wheel pump, is driven via a magnetic clutch. By a rotational drive of the pump being transmitted from a rotary drive member via a magnetic clutch to one of the at least two feed wheels of the pump, the feed wheel nearest to the rotary drive member in the flow of the force, termed the first feed wheel in the following, can be supported independently of the rotary drive member. No mechanical, in particular no positively locking, drive coupling exists between the rotary drive member and the first feed wheel. Possibly occurring, unavoidable friction forces can be assumed to be negligible. In this sense, the first feed wheel is freely rotatable relative to the rotary drive member, aside from the drive coupling produced by the magnetic clutch. In particular, a casing of the pump may form the rotary bearing of the first feed wheel.
The other feed wheel, preferably driven only by the first feed wheel and mating with the first feed wheel to form displacement pockets, is likewise rotatably supported to advantage by the casing. In this way, one and the same rigid body, namely the casing, preferably a single-piece casing part, forms the rotary bearing for the first feed wheel as well as the rotary bearing for the further, second feed wheel. The axes of rotation of the two feed wheels in the pump according to the invention are thus orientated relative to each other more precisely than when the feed wheels are supported on or upon elements moving relative to each other. In particular, the engagement of the two feed wheels with each other can now no longer be disrupted by the change in the loads acting on the rotary drive member, or at least far less than in known pumps. Inaccuracies stemming from assembly are also reduced. The magnetic clutch acts between the rotary drive member and the first feed wheel as a damping member against the transmission of disruptions or irregularities.
The magnetic clutch is preferably configured as a hysteresis or induction-type clutch, or a combination of both. Although less preferred, it is also, however, possible to configure it as a permanently magnetic clutch. The magnetic clutch comprises a magnetic rotating element of a permanently magnetic material in its input half and/or output half. Preferably, the magnetic rotating element is fitted to a soft-iron as a base. A rotating element of the other half of the clutch, producing with the magnetic rotating element the transmission of the magnetic torque, is formed by means of an induction material, or preferably by means of a hysteresis material or a combination of both. An induction material, for example Cu or Al, may form a feedback means and a base for a hysteresis rotating element. However, in such a combined hysteresis/induction clutch, a hysteresis/induction rotating element is preferably likewise fitted to a soft-iron as a base. If the rotating element consists solely of a hysteresis material or solely of an induction material, then a soft-iron likewise advantageously forms the base and the feedback means.
The magnetic clutch may be a face-acting or, more preferably, a centrally-acting rotary clutch. A combination of the two also represents a preferred embodiment.
A gear wheel pump is preferably formed by an internal gear wheel pump or an external gear wheel pump. A gear wheel pump may be formed particularly compactly when the two halves of the magnetic clutch form a central-type rotary clutch, or a combination central/face-type clutch in which the magnetically interacting, concentrically arranged rings encircle the mating feed wheels of the pump, preferably spaced radially from the feed wheels. The combination of an internal gear wheel pump with such a magnetic clutch is of particular advantage.
If the rotary drive member is formed by an input shaft, the first feed wheel preferably encircles the input shaft. However, it is also possible in principle to arrange the rotary drive member and the first feed wheel juxtaposed in the axial direction of the input shaft. In preferred alternative embodiments, the rotary drive member may also be a drive wheel, for example a gear wheel, a sprocket wheel, belt wheel or toothed belt wheel, which then preferably encircles the first feed wheel.
In a particularly preferable internal gear wheel pump, the first feed wheel and the second feed wheel are rotatably supported on or upon circular-cylindrical shell surfaces of the casing, these bearing surfaces preferably encircling each other. The cited magnetic material rings of the magnetic clutch advantageously encircle the two bearing surfaces for the feed wheels.
The invention is not restricted to the field of gear wheel pumps, but also permits advantageous application in the rotational drives of positive-displacement pumps, preferably oil pumps, and in principle pumps of all types. By the drive torque being introduced via a magnetic clutch into the pump, limiting or varying of the delivery, or a combination of both, may be achieved. When a hydrostatic pump or oil pump forms a gear wheel pump, as in preferred embodiments, then the delivery can be limited and/or varied according to requirement by means of the magnetic clutch, without any adjustment to the mating gear wheels of the pump. A variable-delivery external gear wheel pump is known from EP 0 994 257 A1, in which reference is made as an example of this type of pump. However, in a gear wheel pump configured in accordance with the invention, one of the mating gear wheels need to be axially shifted in order to achieve limited and/or varied delivery.
Where only limiting of delivery is required, the magnetic clutch is designed so that once an input half of the magnetic clutch has reached a predefined speed, a limiting torque transmissible by the magnetic clutch and predefined by the designxe2x80x94also described in the following more simply as maximum torquexe2x80x94is attained. If the speed of the input half increases further, the speed of the output half kinks to level off as compared with the speed of the input half. Upon attaining the limiting speed corresponding to the limiting torquexe2x80x94more specifically, the speed correspondingly predefined by the design the speed of the output half preferably remains constant over the speed range of the input half, in operation in excess thereof, or up to a predefined higher speed, as well as this may be approximated due to the magnetic interaction. The maximum torque is dependent on the air gap between the magnetically interacting rotating elements, the shape of the magnetically interacting rotating elements, the magnetically effective materials used, and the dimensions of the magnetically interacting rotating elements, in particular the size of the area collectively covered by these rotating elements of the two halves of the clutch, and a radial spacing of the coverage area from the rotational axis of the clutch. By a suitable selection of materials, dimensions and arrangement of the magnetically interacting rotating elements, the maximum torque of the clutch, and thus the maximum speed of the first feed wheel of the pump, is defined. Other influencing factors, such as for example changes in the viscosity of the pumped medium, affecting the relationship between maximum torque and speed, remains to be taken into account in this consideration. Thus, due to the torque being limited inherently by application of the magnetic clutch, a fail-safe limiting of delivery can be achieved very simply, without the clutch being changed in position, and without any additional means involving the feed wheel of a plurality of the feed wheels. In the case of an engine oil pump, for example, the so-called cold starting valve can thus be eliminated, since the magnetic clutch advantageously acts as a pressure controller, and may even be specifically designed to replace such a pressure control valve.
Limiting delivery may also be achieved by shifting the magnetically interacting rotating elements of the two halves of the clutch relative to each other and as a function of the delivery pressure. Preferably, one of the two halves of the clutch is shiftably supported by the casing of the pump relative to the other half, preferably along the axis of rotation, and such that when shifted relative to the other half of the clutch, the area covered by the magnetically interacting rotating elements of the two halves of the clutch, or a gap between the surfaces facing each other, is changed in size. In this way, the magnitude of the limiting torque as well is automatically changed. In the form of a feedback, the delivery pressure of the pump is placed on the shiftably supported half of the clutch. A spring member or spring-damping member is preferably arranged thereon as a restoring member, so as to counteract the delivery pressure. The magnetic force within the clutch halves, restoring in the direction of full overlap, may be used on its own or in combination with a mechanical or pneumatic spring, to maintain a particular delivery characteristic. A servomotor with an adjustable mechanism is advantageously not used.
The magnetic clutch and the restoring member are, for example, designed such that a delivery characteristic is attained, wherein: the pump exhibits a steep increase in the flow rate and/or delivery pressure, proportional in a first approximation to the speed of the pump, within a first pump speed range; the flow rate is quickly leveled off within a second, higher speed range, up to a preset pump speed; and the flow rate again increases with the pump speed in a third, even higher speed range of the input half of the magnetic clutch, continuing on from the preset pump speed, steeper than in the second speed range, or remains substantially constant in the third speed range. The restoring member can be set as desired, in particular by an arrangement of springs in series.
A delivery characteristic of the aforementioned type may be advantageously used in motor vehicles in which a pump for supplying the motor with it""s lube oil in accordance with the invention is powered by the internal combustion engine of the vehicle, the speed of the pump thus having a fixed relation to the speed of the engine. In the lower engine speedrange, i.e. when starting, vehicles immediately require large amounts of oil. Once a predefined engine speed, and thus the equivalent pump speed and delivery, is attained, no or at least no appreciable further increase in the flow rate of the pump is needed in the speed range continuing beyond the predefined engine speed. Once this medium speed range, in general the main operating range of the engine, has been passed, a high oil flow rate is again required at higher engine speeds, since at higher engine speeds higher centrifugal forces are involved at the points to be lubricated, for example at the crankshaft. Overcoming these increasingly significant centrifugal forces necessitates a higher oil pressure. In general, three speed ranges are to be distinguished in passenger cars; the lower engine speed range from 0 to approx. 1,500 rpm; the subsequent main operating range from approx. 1,500 to approx. 4,000 rpm; and the third, higher engine speed range from approx. 4,000 rpm onwards. To achieve the desired delivery characteristic, namely with a steep increase in the flow rate in the lower speed range, a comparatively slower increase or zero increase in the medium speed range, and finally another steeper increase in the upper speed range, a soft first governor spring is preferably connected in series with a comparatively harder second governor spring. A system of governor springs connected in series is preferably installed pretensioned, such that it hardly gives in the lower speed range. Once the pretension force is passed, as the transition is made between the lower and medium speed ranges, the soft first spring begins to flex until at the upper end of the medium speed range it comes up against the harder second governor spring. With further increase in speed, the characteristic is then determined by the harder, second governor spring.
The design of the clutch, for leveling off the increase in speed of the output half as compared with the input half beyond a limiting speed corresponding to the application in question, may advantageously be employed in combination with an adjustability of the clutch halves, provided for the purpose of changing the transmission characteristic.
The magnetically interacting rotating elements of the magnetic clutch are preferably jointly arranged in the pump casing, such that a temperature equalization of the rotating elements, preferably cooling, is achieved by the medium delivered by the pump. The surfaces of the magnetically interacting rotating elements facing each other particularly preferably face each other directly, and in the preferred arrangement in the pump casing, the medium to be delivered washes around these. In a particularly preferred embodiment, in which the magnetically interacting rotating elements are arranged jointly in the pump casing, facing each other directly, the outer surfaces of the rotating elements are only separated from each other by a thin film of the medium to be delivered.
If the pump is formed with a plurality of feed wheels, these are preferably supported by a rigid casing, preferably a single-piece casing part, not only in gear wheel pumps, but also in other pumps in accordance with the invention, for example worm wheel pumps or wing unit pumps, and not by elements which are relatively mobile with respect to each other, although the latter is not to be excluded in principle.
The two rotating elements of the magnetic clutch are advantageously rotatively mounted by the casing. The two rotating elements of the magnetic clutch are preferably rotatively mounted by the same casing as the first feed wheel or the several feed wheels. The two rotating elements of the magnetic clutch are particularly advantageously rotatively mounted by a single-piece casing. The rotating element of the input half is secured against rotation in its connection to the rotary drive member, but sufficiently mobile to be rotatively mounted by the casing.
A pump in accordance with the invention, when employed as an engine oil pump, in particular in motor vehicles, can be put to use not only as the lube oil pump for the engine and/or an automatic transmission, but may also be used to advantage, for example, for pumping fluid for hydraulic compensation of valve play and/or as a pump for varying valve timing. Application as a feed pump for an automatic transmission or a servo drive, for example a steering servo or in a braking system, is also advantageous.